I wanted to get rid of drone while cruising. My engine turns about 2000 RPM at 70 MPH, which works out to about 130 Hz: (2000 RPM x 4 firing cylinders/revolution)/60 seconds/minute. The approximate wavelength of this sound at 70 mph is 96 inches. I applied this to a discussion I found on a Mustang web site about making a quarter wave closed side tube to set up standing oscillations to cancel out exhaust sound waves. The principle is that a closed-end side tube can be attached to the exhaust (it opens into the exhaust pipe) to provide standing waves that are 180 degrees out of phase with the exhaust waves. Being 180 degrees out of phase, the troughs in the standing wave will correspond to the peaks in the exhaust waves (and vice versa) which tends to cancel out the sound with no increase in back pressure. The side tube needs to be ¼ of the exhaust wave length because sound waves entering the tube will reflect off the closed end to give an effective path length of 1/2 the wave length, which is 180 degrees out of phase. So, for the 2000 RPM situation, the tube needs to be about 24 inches long. For maximum sound control, the closed tubes need to be the same diameter as the exhaust pipe. I built two rear exhaust pipes with side tubes to replace my resonators. My side tubes are only 22 inches long because I built the assemblies in NJ (where my tools and welder are) but my car is in WI (where I am living now) and I had to estimate what would fit. It turns out there is plenty of room to make longer tubes… Also, I used 2 inch tubing for the side tubes versus 2 1/4 inch tubing for the exhaust because of the tighter bends available in 2 inch tubing. Still, the tubes work remarkably well. There is some minor drone at 60 mph, but it is gone at 70 to 85 mph (and probably higher, but I have not driven that fast yet). Using fourth gear at 70 mph (3000 RPM) there is some slight noise, but nothing like it would be without the modifications. Noise under hard acceleration is also quieter over the whole range and seems somehow smoother. This is to be expected. While the tubes will give the most noise cancellation at a specific frequency, they will still provide some attenuation over a much wider range until the standing wave reaches the same frequency as the main exhaust frequency. So, to summarize, I removed my resonator glass packs and replaced them with open pipes, yet the sound is significantly decreased. Picture is attached.

My exhaust system has a crossover pipe; without it, each exhaust will see only 2 firing pulses per engine revolution, which would make the frequency too low (at 2000 rpm) for a practical resonator pipe length.

I put my resonator pipes at the end of the exhaust system because that was the only place where there was room. But it works at just about any point on the system, and it would be interesting to see what would happen if the resonator pipes were placed before the main mufflers.

There is some speculation that a chambered muffler (not a glass pack) can act as a sound reflector so that the pipe from the muffler to the end acts as an organ pipe with a tuned frequency appropriate for the pipe length. In small sports cars, this distance tends to be rather short (in my case 60 inches) so the resonate frequency would come in at a point well above cruise RPM. But if it is a problem, the resonator pipe can be made ¼ the length of this pipe section to kill any tendency for amplification at the natural pipe frequency.

I saw where one person incorporated small glass pack mufflers into the resonator pipes with the intention of making the resonators cover a wider range of frequencies. I am not sure if this will work; the muffler may just act to kill or weaken the standing wave.

The pipes are relatively easy to build and experimentation is always encouraged. I have included a picture of a pipe as removed from the car to give a better indication of the construction. Note that the upper (short pipe) has a closed end; this may not be clear in the photo.

If you dare, when elswhere is no room, perhaps roll cage or unit body tube structure ? In other words, a cap plate could be welded in, to tune the length ? A flex or swivel joint and heat insulation might be needed. Onward, roverman.

What about a crossover that has a tuned stub somewhere close to the middle? Also, is the resonance a function of the distance traveled by the standing wave, or a function of the volume? If the latter, wouldn't a diameter increase effectively shorten the tube?

The resonance is a function of the length; while the intensity is a function of volume (diameter).Think of how a pipe organ works...

I received a private post from a Mustang owner who says he eliminated drone by installing different sized mufflers on each side of the dual exhaust system (note that all Mustangs are factory equipped with cross-over pipes)..

This post has had my attention since Larry posted it. About two months ago, I had my neighbor's 2004 Honda, S2000 on my lift to help him change the oil; while the oil was draining, we've looked over the undercarriage. I was amazed at the high tech engineering of the car and while looking at the exhaust I observed two short pipes sticking out the side of each pipe running to the back (4 banger with dual exhaust). The pipes were about 6 inches in length and were mounted at about the center of the car.

If the pipes were connected together, you would think that it was an "H" pipe, however, the pipes stood well apart by themselves on each run and were capped. I concluded that they were exhaust tone tuners, not quite sure.

On my MGB built with a SBF 302 stroker (331), I plan in using two back mounted stainless mufflers with spiral technology (almost new); there's a weight saving and exhaust flow increase factor. Since I also plan in using two high-flow catalytic converters, I assume that it would tone the loudness down a bit.

I've purchased the pair used from a 2010 Mustang with a 5.0 Liter "Boss" modular engine. The Mustang owner stated that he wanted to sell them because they were too loud for his taste; based on his statement my question is; would a pipe (resonator tube) such as in Larry's posting, help in designing a desirable sound and loudness? your input would be appreciated.

Just did a search, and found this discussion. I thought I remembered Larry Shimp doing something to his exhaust, and this is the article.
My 302 in my TR6 has separate "resonator" mufflers (I expect just a straight glasspak) just off the headers, with no H pipe. At the rear, the
two 2 1/2" exhaust pipes go into a two in, two out Magnaflow. The drone at 2200 is unpleasant, and would be easy to work around if it was only
a cruise problem, but it occurs in every gear as I work up to speed. I will discuss this idea with the muffler shop, and see what happens.
I also need to look into a chambered muffler.
On another note, I ran a data log on my Megasquirt, and running up in 2nd and 3rd, I see 19.5ms injector PW. Am I in need of bigger injectors?
Have not done the dyno tune yet, I wanted to have the exhaust worked out first.
Dennis

Some of this conversion business takes a lot of time! I'm still displeased with the drone in my exhaust, but have space issues.
Jim mentioned resonator tube perpendicular to an H pipe. I'm thinking that having an H tube just at the back of the transmission, where the tubes are closest, with a resonator connected to both exhaust pipes might be a solution. Do you think the cavity resonance formula would result in a tube that would damp the resonant frequency?
Dennis

On my 6-pot TR6, I had problems with drone/resonance. Ended up swapping out a twin system for a single system.

On my Land Rover Lightweight (215 V8) there was a good home-brew mild steel exhaust, loud but rusted until it fell off, then I had a custom stainless system that had horrible boom and became very loud when the glass blew out, then I changed to a glasspack Flowmaster that has no drone but sounds like a Gatling gun, now I'm going to a Hooker Aerochamber that I hope will give the boom without the bark. Single box, 14" long.
[www.jegs.com]

Regarding the crossover, what I'm thinking, is adding one and putting the resonance chamber "T'd" in the center of the crossover.
The resonator should still see 4 pulse per cycle, keeping the 1/4 wave short enough. My current install is separate pipes all
the way back to the Magnaflow "X" muffler in the rear. I was talked into this, and I'm not convinced it was ideal. I'd replace the whole thing, but it was pricey.
Dennis

I am still satisfied with my resonator tubes, but I am interested in how the crossover resonance tube will work.

By the way, I repacked my Magniflow mufflers several years ago. All of the original packing had blown out. I cut a round hole in one end with a hole saw and put in packing made for dirt bike mufflers; it had much longer strands than the original and so should last longer. I closed the hole with an electrical junction box blank off that I bought from McMaster Carr. This will make it easy to replace the packing again if needed.